1. Field of the Invention
This invention relates to a light deflector for scanning with light from a light source and an image forming apparatus that utilizes such a light deflector. For example, the present invention can suitably be applied to an image forming apparatus such as a projection display adapted to project an image by deflecting light for scanning or a laser beam printer or a digital copier that involves an electro-photographic process.
2. Related Background Art
Various optical scanning systems (and optical scanning apparatus) that utilize a reflection surface to be subjected to sinusoidal oscillation so as to make it operate as a light deflector have been proposed to date. Optical scanning systems that utilize a reflection surface to be subjected to sinusoidal oscillation with use of a light deflector using a resonance phenomenon provide advantages. These advantages include that the light deflector of such an optical scanning system can be remarkably downsized, that they consumes little electric power and that a light deflector formed by Si single crystals that are manufactured by way of a semiconductor process is theoretically free from metal fatigue and highly durable, as compared with an optical scanning system that utilizes a rotating polygon mirror or the like.
As for light deflectors that utilize a resonance phenomenon, known optical scanning techniques for simultaneously exciting two or more than two resonance modes in different senses of rotary oscillation include the following. Firstly, there is known the use of a gimbal-type light deflector that realizes two-dimensional deflection by simultaneously exciting two resonance modes in two senses of rotary oscillation around central axes that are orthogonal relative to each other. Secondly, there is known the use of a light deflector that operates for scanning at a substantially same angular velocity like a triangular wave by simultaneously exciting two or more than resonance modes around the same central axis.
FIG. 9 of the accompanying drawings schematically illustrates a typical gimbal-type light deflector. Referring to FIG. 9, an outer movable element 4023 is connected to a support body 4025 by means of a pair of first torsion springs 4024 so as to be able to give rise to torsional oscillation that takes place around a first axis of torsion 4017. On the other hand, a movable element 4021 is connected to the outer movable element 4023 by means of a pair of second torsion springs 4022 so as to be able to give rise to torsional oscillation that takes place around a second axis of torsion 4018. A light deflector (not shown) such as a reflection surface that deflects light is arranged on the movable element 4021. Therefore, it is possible to two-dimensionally deflect light from a light source for scanning as the outer movable element 4023 and the movable element 4021 are driven to give rise to torsional oscillation respectively around the first axis of torsion 4017 and the second axis of torsion 4018 by a drive means 4016. The drive means 4016 may be formed by using permanent magnets arranged respectively at the movable element 4021 and at the outer movable element 4023 and fixed coils for driving the respective permanent magnets. This arrangement is advantageous particularly when the light deflector is driven with the frequency of natural oscillation mode around the axis of torsion that is defined by the profile and the material of the light deflector because a large displacement is produced by a small amount of energy.
A two-dimensional scanning system that is compact and simple for optical adjustments can be realized by using such a gimbal-type light deflector because a single light deflector can operate for two-dimensional deflection/scanning. U.S. Pat. No. 6,044,705 and Japanese Patent Application Laid-Open Nos. H07-27989 and 2003-295102 describe gimbal-type light deflectors of the type under consideration.
FIG. 8 of the accompanying drawings schematically illustrates a light deflector that operates for scanning at a substantially same angular velocity like a triangular wave by simultaneously exciting two resonance modes.
Referring to FIG. 8, light deflector 1012 is formed by a first movable element 1014, a second movable element 1016, a fist torsion spring 1018 for connecting them and resiliently supporting them and a second torsion spring 1020 for resiliently supporting the second movable element 1016 and a mechanical grounding surface. All these components are driven by a drive means 1023 to give rise to torsional oscillation around an axis of torsion 1026. The first movable element 1014 has a reflection surface 1015 for deflecting light so that it deflects light from a light source by torsional oscillation for scanning. The light deflector 1012 has a primary natural oscillation mode that provides a reference frequency and a secondary natural oscillation mode that provides a frequency equal to about three times of the reference frequency for torsional oscillation around the axis of torsion 1026. The drive means 1023 drives the light deflector 1012 at two frequencies, one equal to that of primary natural oscillation mode and the other that is in phase with and equal to about three times of the first one. Thus, since the light deflector 1012 gives rise to torsional oscillation simultaneously in a primary natural oscillation mode and also in a secondary natural oscillation mode, the displacement angle of light reflected by the first movable element 1014 changes not like a sinusoidal wave but like a triangular wave due to the two oscillation modes that are superposed one on the other. Therefore, the angular velocity of the deflecting/scanning operation of the light deflector shows a wide range where it remains substantially constant to give a large ratio of the usable range relative to the overall deflection/scanning range if compared with the displacement angle that changes like a sinusoidal wave.
U.S. Pat. Nos. 4,859,846 and 5,047,630 describe light deflectors of the above-described type.